Although the idea of enabling D2D communications as a means of relaying in cellular networks was proposed by some early works on ad hoc networks, the concept of allowing local D2D communications to reuse cellular spectrum resources simultaneously with ongoing cellular traffic is relatively new. Because the non-orthogonal resource sharing between the cellular and the D2D layers has the potential of reuse gain and proximity gain, while at the same time increasing resource utilization, the use of D2D communications underlying cellular networks has received considerable interest in the recent years.
The Third Generation Partnership Project, 3GPP, has introduced D2D communications for Long Term Evolution, referred to as “LTE Direct”. These types of D2D communications can be used in commercial applications, such as cellular network offloading, proximity based social networking, or in public safety situations in which first responders need to communicate with each other and with people in the disaster area. See the Technical Report identified as 3GPP TR 22.803, studying the feasibility of proximity services, “ProSe”, using D2D communications.
D2D communication entities that use an LTE Direct link may reuse the same physical resource blocks, PRBs, as used for cellular communications either in the downlink or in the uplink or both. Those of ordinary skill in the art will recognize that PRBs comprise time/frequency resources allocable for multi-user service. The reuse of radio resources in a controlled fashion can lead to the increase of spectral efficiency, however such reuse comes at the expense of some increase in intra-cell interference.
Typically, D2D-communicating entities use uplink, UL, resources such as UL PRBs or UL time slots. However, it is possible that D2D communications, such as LTE Direct communications, take place in the cellular downlink, DL, spectrum or in DL time slots, and the teachings disclosed herein apply for UL and DL reuse. However, for ease of presentation, this disclosure assumes that D2D communication links use UL resources. For example, it is assumed that D2D communications reuse UL PRBs from a frequency-division duplexing, FDD, cellular communication system. For a time-division duplexing, TDD, cellular communication system, the D2D entities reuse UL time slots.
FIG. 1 illustrates a communication network 10 in which spectrum used by the network is reused for D2D communications, e.g., a frequency band or carrier frequency f0 is used by the network 10 and is reused at least partially for D2D communications. The network 10 has a coverage area 12 having a coverage border 14. While the network 10 may have multiple, overlapping coverage areas or cells, the diagram illustrates a single coverage area as a simplified basis for illustrating the principles of D2D communication within the LTE context.
A controlling node 16—such as an eNodeB or Cluster Head, CH—controls communications on a carrier having a frequency f_0. Here, a “CH” is a wireless device, such as a 3GPP user equipment, UE, which has control over a number of other wireless devices and acts as a base station with respect to those other device. However, the CH does not necessarily have a connection to the Internet or to a cellular system. In the diagram, wireless devices 18-1 and 18-2 communicate directly via a D2D link, while the wireless device 18-3 is not engaged in D2D communications. As both devices 18-1 and 18-2 are inside network, NW, coverage of the controlling node 16, the controlling node 16 allocates the radio resources used for D2D communication between the device 18-1 and 18-2.
In contrast, the wireless devices 18-4 and 18-5 communicate via D2D communication, but they are out of NW coverage. As a consequence, the devices 18-4 and 18-5 use pre-configured time/frequency resources, e.g., as pre-configured according to the controlling standards and/or based on device capabilities. In general, D2D communications may be “assisted D2D” that is carried out inside network coverage with the usual 3GPP meaning, or coverage from a cluster head, or may be “non-assisted” or “self-contained” D2D that is conducted outside of network coverage or coverage from a cluster head.
To maximize the coverage of D2D communication in NSPS scenarios, it is currently assumed that an NSPS device 18 may use higher output power than an ordinary LTE device 18 on dedicated NSPS frequency bands. The area in coverage of the NSPS base station is here termed an NSPS cell. Thus, in the example of FIG. 1, the coverage area 12 may be an NSPS cell, and the controlling node 16 may be an NSPS base station. Correspondingly, the proposed device transmission power, TX power, for NSPS operation is 31 dBm, as compared to the 23 dBm in the current cellular LTE standard.
Because the power amplifiers, PAs, used in such devices 18 typically are non-linear for high output powers, there will be spectral leakage—non-linear effects—outside the desired frequency band. FIG. 2 shows an example where a given device 18 transmits at a carrier frequency f0 at high power. Due to spectral leakage there will be emissions in adjacent and close frequency bands, e.g., affecting a carrier frequency f1. The carrier frequency f1 is considered to be a critical frequency band if it is standardized or otherwise known for use as a communication band, such as a cellular band, or an emergency, satellite, terrestrial broadcast, radar or military communications band. If one now assumes that there is active transmission on the f1 carrier, the spectral leakage may significantly interfere with that communication. Active transmissions in a critical frequency band will be referred to as critical transmissions.
See, for example, FIG. 3, which illustrates a cellular network 20 using a carrier frequency f1 that is adjacent to a carrier frequency f0 used for D2D communications. Here, “adjacent” may be bordering or immediately next to, but more broadly means close or nearby. The cellular network 20 provides one or more coverage areas 22 over which cellular communication services are provided on the f1 carrier frequency.
To the extent that devices 18 operating on a D2D link on the f0 carrier frequency are nearby or inside the coverage boundary 24 of the cellular communication network 20, the D2D transmissions may substantially interfere with the cellular transmissions between a base station 26 of the cellular communication network 20 and a wireless device 26 connected to that network 20. For example, FIG. 3 depicts wireless devices 18-4 and 18-5 communicating on a D2D link at the f0 carrier frequency and operating within the coverage area of the cellular communication network 20. Out-of-band leakage from the D2D transmissions causes interference in the f1 carrier frequency.
In one example relevant to the United States, 3GPP, NSPS is standardized for use in 3GPP Frequency Band 14—UL at 788 MHz-798 MHz—while ordinary, non-NSPS cellular communication may be deployed in 3GPP Band 13—UL at 777 MHz-787 MHz. Thus, the wireless devices 18-4 and 18-5 may have a D2D link in the 788-798 MHz frequency band, while ordinary cellular communication uplink between the base station 26 and wireless device 28 may be in the 777-787 MHz frequency range.
Conventional D2D transmission schemes do not provide interference mitigation with respect to critical frequency bands that are vulnerable to, e.g., out-of-band interference arising from D2D transmissions. Nor do existing D2D transmission schemes even provide a basis for recognizing when there are such interference risks. For example, it is recognized herein that with respect to the above scenario, there is only 1 MHz of separation between the lower extent of the D2D frequency band and the upper extent of the cellular frequency band. Thus, the cellular frequency band is “adjacent” in the sense that it is close to or nearby the D2D frequency band. Furthermore, there typically will be no coordination between emergency NSPS usage in Band 14 and ordinary cellular usage in Band 13. Hence significant uplink interference can be induced at the cellular radio base stations, which in turn may jeopardize the uplink coverage of the cellular system.